A variety of spectroscopic equipment has been developed for analyzing the absorption and transmission of electromagnetic radiation by a sample at various wavelengths to determine chemical characteristics of the sample. One type of spectroscopic instrument now widely used is a Fourier transform infrared (FTIR) spectrometer. FTIR spectrometers typically incorporate a Michelson interferometer having a moving mirror. The interferometer modulates the infrared beam from an infrared source to produce an output beam in which the intensity of the infrared radiation at various wavelengths is periodically varied. The output beam is typically focused and passed through, or reflected from, a sample, after which the beam is collected and focused onto a detector. The detector provides a time varying output signal which contains information concerning the wavelengths of infrared absorbance, or specular reflectance, of the sample. Fourier analysis is performed on the output signal data to yield usable information on the chemical composition of the sample.
It is often desirable to do spectroscopic analyses on gases and liquids, including flowing gases and liquids, and, in some cases, on solids. In many of these situations, it is not convenient or desirable to remove a sample of the material and place it in the sample chamber of the spectrometer for analysis.
One approach to interfacing the beam of analytical radiation from the spectrometer with a liquid or gas sample is the use of an attenuated total reflectance (ATR) probe. In ATR devices, the optical element is shaped so that radiation enters through one face of the element, passes along the element, and exits through another face. Between the entrance and exit, the radiation makes a number of total internal reflections from the sidewalls of the element. The sidewalls of the element are in contact with a liquid or gas sample, and as a consequence of the physical phenomenon of internal reflectance, the analytical radiation is selectively attenuated at certain wavelengths characteristic of the sample material in contact with the exposed face of the ATR optical element. A variety of designs for optical elements for ATR probes are described in the book by N. J. Harrick, Internal Reflection Spectroscopy, Harrick Scientific Corporation, New York, 1979.
The optical elements for ATR probes are commonly crystals having highly polished flat surfaces which interface with the sample. One type of prior probe uses a cylindrical ATR crystal having a conical end which acts as the interface with the sample. Such optical elements must be precisely polished and thus are generally expensive to produce and expensive to replace if the polished surfaces become scratched or damaged. Where ATR cells are used at a location remote from the spectrometer, optical fiber cables are sometimes used to transmit the incoming beam to the ATR probe and to return the beam from the probe to the spectrometer. Focusing optics are generally required to receive the radiation from the end of the first fiber optic cable and direct it through the input face of the ATR optical element. The analytical radiation passed from the output face of the ATR optical element must then be collected and focused onto the end of the optical fiber cable connected to the spectrometer. The need for such focusing optics makes assembly of such ATR probes more difficult and expensive. Typically, ATR probes are also relatively large because of the need for focusing optics and the relatively large ATR optical element. These probes are also difficult to use with commercial tube fittings commonly found in process lines and chemical reactors, and typically require at least 1/4 inch diameter fittings.